Skin loss due to burns and ulcers is a major medical problem. In the United States, it is estimated that 1.25 million people need medical care for burns each year. Approximately 51,000 people are hospitalized for burns and 13,000 require skin grafts. Of these, 1,500 are severely burned and require extensive grafting. With respect to skin ulcers, it is estimated that 800,000 diabetics in the United States suffer from chronic, non-healing diabetic foot ulcers. Infections of some of these skin ulcers result in 50,000 amputations each year. In addition to diabetic ulcers, new products are needed for venous ulcers, and pressure ulcers (bedsores).
Several products employ bioengineered skin tissue, some of which include cultured keratinocytes with or without a dermal component. In patients with large total body surface area full-thickness burns, the burned skin is first removed to reduce toxicity from degradation of the necrotic tissue and to reduce microbial growth in the wound. Excised wounds are covered temporarily with cadaver skin to reduce fluid loss and infection (Boyce et al., J. Burn Care Rehab. 20(6):453-461 (1999)). The culture methods of Rheinwald and Green have allowed sheets of autologous keratinocytes to be used in the clinic since 1985. However the lack of a dermal component in some of these products, e.g., Genzyme's Epicel, limits their utility. Other products consist of only a dermal analog, e.g., Advanced Tissue Science's Transcyte and Dermagraft and LifeCell's Alloderm. These products currently are not as effective as split thickness autologous grafts, which remain the method of choice for the treatment of full-thickness skin loss. However autologous grafts may not be possible in patients with extensive burns and may even in other patients create wound-healing problems at the autograft donor site including opportunities for infection and the generation of functional or cosmetic defects.
The organotypic culture technique for normal keratinocytes has fostered the recent development of cultured skin tissue for the treatment of burns and skin wounds. Composite grafts consist of keratinocytes seeded onto dermal analogs, e.g., a fibroblast-contracted collagen in the Apligraf product from Organogenesis. Composite grafts, however, are reportedly more fragile and slower to revascularize. The use of cultured skin tissue for burn or wound therapy can therefore be limited by poor vascularization of the graft. In autografts, vascularization occurs by inoculation and neovascularization. Inosculation proceeds by the joining of capillaries in the wound bed to ends of the severed vessels in the dermis of the graft. This is the primary mechanism of early vascularization. Vascularization of composite grafts takes longer than an autograft because it relies only on neovascularization. The poor vascularization of composite grafts leads to graft ischemia and a delayed time to successful engraftment.
Because of the limitations of current bio-engineered tissues, there is a great deal of opportunity for improved products for the treatment of burns and skin wounds. Clearly, a great need exists for cultured skin tissue having improved properties for transplantation.